Average Follow-up Time DetailWe estimated average person-years of follow-up by multiplying the reported average follow-up time by the number of persons.

Exposure DetailHistory of head injury was ascertained twice for participants in this study. First, any head injury resulting in a loss of consciousness was ascertained by a physician interviewer. In a separate interview, head injury resulting in a loss of consciousness or amnesia was ascertained by a trained risk factor interviewer. Participants who confirmed a history of head injury through the risk factor interviewer were asked further questions on when the injury occurred and the duration of any loss of consciousness, which was recorded as "< 5 minutes, 5-29 minutes, 29-59 minutes, 1-24 hours, or > 24 hours. A report of head injury with amnesia but no loss of consciousness, would lead to classification as head injury with loss of consciousness< 5 minutes by the risk factor interviewer, but that same history would be recorded as no head injury with loss of consciousness by the physician."

The last two categories of head injuries include participants who also make up the second and third categories.

The investigators examined any head injury with loss of consciousness as well as severity and timing of head injury. This entry is for any head injury by timing.

Total dementia definition: "Dementia was diagnosed by means of a strict algorithm which required that patients met threshold criteria on neuropsychological evaluation, had evidence of functional impairment either by history or examination, and that the cognitive problems could not be attributed to an acute confusional state. The neuropsychological criteria for dementia were performance below previously defined cut off scores in two out of three memory domains, and in at least two tests of othercognitive domains.(10)"